The present invention relates to a method of anisotropic etching of structures in a semiconductor body, in particular of recesses in a silicon body exactly defined laterally by an etching mask, by using a plasma according to the definition of the species of the main claim.
German Patent 42 41 045 C1 describes a method of anisotropic plasma etching of silicon. In this method passivation and etching steps are alternated with polymer material being deposited on the structures exposed by etching during the passivation steps and fluorine radicals, which etch silicon anisotropically per se, being released from a fluorine-delivering agent in the plasma during the etching steps. At the beginning of each etching step, breakdown of the etching base produced in the previous passivation step is first completed by a controlled ion bombardment of the etching base of the etched structures during the etching steps.
During the essentially isotropic etching steps, local anisotropy is achieved by the fact that the portion of obliquely incident ions hitting the side wall of the structures effect forward sputtering during etching, i.e., stripping and redeposition of the side wall polymer film toward the deeper parts of the trench produced, so that the side walls always remain passivated by a protective film as etching continues and are protected against the etching attack of the isotropically acting fluorine chemicals.
For this purpose the ion energy needed during the etching steps, which is produced via an acceleration voltage generated by injecting high-frequency power and applied to the substrate electrode, i.e., the silicon wafer to be etched, must be sufficient to ensure breakdown of the etching base passivation at the beginning of each etching step, while keeping the etching base free of polymer during the etching step and maintaining the side wall polymer film transport mechanism. This is achieved in German Patent 42 41 045 C1 through a relatively low ion energy of 10 to 15 eV which is applied for the entire duration of the etching steps.
Due to the constant ion acceleration, constant selectivity is obtained between side wall film stripping and the etching base polymer stripping, the duration of each etching step being limited by the fact that most of the side wall polymer film has been stripped away, i.e., forward sputtered, during the etching steps. Therefore it must be periodically reinstalled using a new passivation step, which ultimately also limits the etching rates that can be achieved, because the passivation pauses represent an interruption of the etching process. If the process is run with too little passivation, anisotropy of the etching suffers and lateral pockets are formed by a phenomenon known as xe2x80x9cnotchingxe2x80x9d in the area of the dielectric boundary surfaces, i.e., for example, at the etching stop layer when silicon etching is stopped.
In unpublished German Patent Application 198 41 964.3 it is proposed that a plasma etching system be provided with a device for cooling a silicon body during an anisotropic etching process. Finally, it is known from German Patent Application 197 30 644.9 that the concentration of fluorine atoms or fluorine ions can be determined in a plasma using an optical emission spectrometer and a downstream analyzer.
The method according to the present invention having the characterizing features of the main claim has the advantage over the related art that it allows adjustment of the intensity of the effective ion acceleration between plasma and substrate electrode to the physical-chemical processes during the etching steps and, when needed, also during the polymerization steps, so that the side wall polymer film can withstand the etching step for a longer period. This results in that longer etching times can be used in the etching steps and interruptions by passivation steps become less frequent, i.e., they are only required at longer time intervals. Therefore, a higher overall etching rate is advantageously achieved.
Advantageous refinements of the present invention are derived from the measures named in the subclaims.
Thus, the mask selectivity is advantageously improved by the fact that etching is performed using a high ion acceleration voltage only in the initial etching base polymer breakdown phases, while during the remaining duration of the etching steps only the etching base of the polymer material is kept free and the side wall film transport mechanism is preserved through reduced ion acceleration produced by reduced voltage.
Finally, anisotropy of the etching process, in particular with regard to the vertical profile achieved, is also improved, because side wall passivation is more effective and the process parameters can be optimized so that always the exactly correct amount of polymer is stripped and redeposited, i.e., forward sputtering is achieved through ion bombardment induced by the voltage applied.
It is furthermore advantageous that due to a temporarily reduced high-frequency AC voltage or injected high-frequency power, which is associated with a reduced ion acceleration voltage, during the etching steps the side wall polymer will be stripped more slowly, so that better selectivity of the etching base polymer stripping relative to the side wall polymer stripping is achieved over an etching step. As a result, the etching steps can be very advantageously extended in time which improves both the etching rate and at the same time also etching anisotropy. This is explained by the fact that in the etching steps exactly as much polymer is forward sputtered as is needed for achieving an optimum etching profile (vertical walls). At the same time, with reduced ion acceleration also less mask material is stripped, also advantageously achieving higher mask selectivity.
Finally, the method according to the present invention is also xe2x80x9csofterxe2x80x9d overall, i.e., less corrosive on reaching a dielectric boundary surface, because the ion energy is reduced each time after the breakdown of the etching base passivation. Harmful effects of the ion bombardment are thus reduced, in particular the xe2x80x9cnotching effectxe2x80x9d with formation of pockets as a result of electrical charges and ion deflection in the boundary surface areas over the remaining time of the etching step.
It is furthermore advantageous that the duration of the etching segments can be dynamically controlled, via an optical emission spectrometer which determines the fluorine concentration in the plasma as a function of time, and regulated via the high-frequency generator.
The method according to the present invention furthermore allows to advantageously vary the injected high-frequency power continuously with a ramp function between the etching segments and on transition from etching steps to polymerization steps.
Furthermore, it is advantageous that the method according to the present invention can be combined with a plurality of additional process parameters and optimized to the respective requirements. The process pressure, the gas flow rate of the reactive gases supplied, and the plasma power, for example, are particularly suitable for this purpose.
It is also particularly advantageous if the variation over time of the ion acceleration voltage is combined with an adjustment of the temperature of the silicon body to be etched. This allows almost simultaneous breakdown of the etching base polymer to be achieved regardless of the aspect ratio of the respective trenches.
In order to modify the ion acceleration voltage, a plurality of options are advantageously available. The preferred method includes modulation of the injected high-frequency power with a low-frequency modulation voltage during the etching segments or dividing the high-frequency power or high-frequency AC voltage applied into AC voltage pulses with a defined adjustable pulse/pause ratio, which in a particularly advantageous embodiment is considerably smaller than 1. Thus electrical charges capable of causing xe2x80x9cnotchingxe2x80x9d effects can be reduced.
The ion acceleration voltage is obtained in this case in the form of very short, very intensive high-frequency pulses with relatively long pauses between the pulses, the average value of the ion acceleration being adjustable directly via the pulse/pause ratio in a broad range.
In order to accelerate the initial etching base polymer breakdown at the beginning of an etching step, it is furthermore advantageous if a gas that is chemically corrosive particularly toward the etching base polymer such as NF3 or O2 is added during this time.